Composite optical element, optical apparatus and imaging apparatus

ABSTRACT

A composite optical element comprises a first base member, an optical resin layer, a bonding layer, and a second base member which are sequentially laminated such that the optical resin layer and the bonding layer are sandwiched between light entering/exiting surfaces of the first base member and the second base member. The thickness of the bonding layer changes along a straight line extending from the center toward the outer periphery of the bonding layer. Specifically, the thickness along the straight line is greater at an intermediate position between a first position and a second position than either of the thicknesses at the first position and at the second position. The first position is apart from the center by 0.8 times of half the diameter of the optical resin layer, and the second position corresponds to the outer periphery of the bonding layer.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a composite optical element formed bycompositely combining one or more glass-made (or hard plastic-made) basemembers and resin-made members. More specifically, the present inventionrelates to a composite optical element comprising a pair of basemembers, an optical resin layer and a bonding layer, the optical resinlayer and the bonding layer being arranged between the pair of basemembers.

Description of the Related Art

In recent years, in response to a demand for imaging apparatus such ascameras and video cameras with improved performances, optical elementssuch as lenses for forming optical devises equipped with an imagingoptical system are required to exert improved optical functions and showexcellent environmental resistance. A known technique of arranging anadditional lens in the optical system in order to correct the chromaticaberration that arises due to the use of spherical lenses is beingpopularly employed in instances where the optical system of an opticaldevise is formed by using a plurality of lenses.

Japanese Patent Application Laid-Open No. 2010-117472 discloses acomposite optical element formed by sequentially laying a first opticalelement (which is a glass base member), a resin layer formed on thelight-entering/exiting surface of the first optical element showing aconvex profile, a second optical element (which is a glass base member)having a light-entering/exiting surface showing a concave profile andbonded to the resin layer by way of a bonding layer so as to operate asa lens for correcting chromatic aberration. The resin layer has a filmthickness that is uneven and largest at the center and continuouslydecreases toward the outer peripheral edge thereof. The effect ofcorrecting chromatic aberration of a resin layer having an uneven filmthickness rises as the ratio of the largest film thickness at the centerto the smallest film thickness at the outer peripheral edge, which ratiois referred to as uneven thickness ratio, is increased. Thus, there is ademand for optical elements having a resin layer that shows a largeuneven thickness ratio.

However, in a composite optical element disclosed in Japanese PatentApplication Laid-Open No. 2010-117472, its components including a pairof glass base member, a resin layer and a bonding layer are made ofdifferent materials and hence their linear expansion coefficients andmoduli of elasticity differ from each other. Therefore, as thetemperature of the environment surrounding the composite optical elementchanges, internal distortion arises due to the differences among theirthermal expansion coefficients. Particularly, when the resin layerhaving a large thermal expansion coefficient relative to the glass basemembers shows a large uneven thickness ratio, the internal distortionwill become remarkably large in response to changes in the environmentaltemperature. Therefore, an optical element disclosed in Japanese PatentApplication Laid-Open No. 2010-117472 is accompanied by a problem thatthe resin layer and the bonding layer become liable to come off fromeach other when the environmental temperature changes to a large extent.

In view of the above-identified problem, Japanese Patent ApplicationLaid-Open No. 2016-194609 discloses a composite optical element in whichthe internal distortion that arises between the resin layer and thebonding layer is reduced by reducing the thickness at an outerperipheral part (outer peripheral edge zone) of one of the pair of glassbase members to thereby improve its thermal resistance of the compositeoptical element.

However, while a composite optical element disclosed in Japanese PatentApplication Laid-Open No. 2016-194609 can minimize the risk ofseparation of the resin layer and the bonding layer by raising theflexibility of the base member, the outer peripheral part (outerperipheral edge zone) of the base member having a reduced film thicknessbecomes an optically ineffective region in the optical system to whichthe composite optical element belongs. This means that the compositeoptical element is inevitably made to have a large outer diameter.

The present invention is made in view of the above-described technicalbackground and provides a composite optical element comprising a pair ofbase members, an optical resin layer and a bonding layer, the opticalresin layer and the bonding layer being arranged between the pair ofbase members, that shows an excellent environmental resistance even whenits outer diameter is reduced.

SUMMARY OF THE INVENTION

A composite optical element according to the present inventioncomprises: a first base member having a light entering/exiting surfaceextending from a center, the center being aligned with the optical axisof the optical element, to an outer periphery thereof; an optical resinlayer extending from the center to an outer periphery thereof; a bondinglayer extending from the center to an outer periphery thereof; and asecond base member having a light entering/exiting surface extendingfrom the center to an outer periphery thereof; the first base member,the optical resin layer, the bonding layer and the second base memberbeing sequentially laminated in the above-described order; the opticalresin layer and the bonding layer being sandwiched between the lightentering/exiting surface of the first base member and the lightentering/exiting surface of the second base member, wherein the bondinglayer has a thickness which changes along a straight line extending fromthe center toward the outer periphery of the bonding layer, thethickness being greater at an intermediate position located between afirst position and a second position than at the first position and atthe second position, the first position corresponding to a point apartfrom the center by 0.8 times a distance to the outer periphery of theoptical resin layer from the center, the second position correspondingto the outer periphery of the bonding layer.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a composite opticalelement according to the present invention.

FIG. 2 is a schematic cross-sectional view of an outer peripheral partof a composite optical element according to the present invention.

FIGS. 3A, 3B, 3C, 3D and 3E are a schematic illustration of a method ofmanufacturing a composite optical element according to the presentinvention, showing different manufacturing steps thereof.

FIG. 4 is a schematic cross-sectional view of an imaging apparatusformed by using one or more composite optical elements according to thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

In a composite optical element, the materials of individual componentsincluding glass-made (or hard plastic-made) base members, an opticalresin layer and a bonding layer differ from each other and hence thelinear expansion coefficients and the moduli of elasticity of thesecomponents differ from each other. More specifically, the optical resinlayer and the bonding layer are made of resin materials whose linearexpansion coefficients show large values and whose moduli of elasticityshow small values if compared with the base members that are made ofglass (or hard plastic). Therefore, as the environmental temperaturechanges, while the deformations of the parts of the optical resin layerand those of the bonding layer that are located close to the interfaceswith the base members are suppressed by the base members having a smalllinear expansion coefficient, those of the parts that are located remotefrom the interfaces with the base members are majorly dominated by theirown linear expansion coefficients. Thus, largely deformed parts andslightly deformed parts coexist in the optical resin layer and also inthe bonding layer that are sandwiched between the two base members togive rise to internal distortion. Additionally, if the optical resinlayer shows a large uneven thickness ratio, the stress produced at thecenter and the stress produced along the outer periphery thereof show alarge difference.

When the stress is observed in the normal direction on the interfacebetween the optical resin layer and the bonding layer, there existcompression stress that forces the two layers to come closer to eachother and tensile stress that forces the two layers to go apart fromeach other. Of these stresses, if the tensile stress exceeds theadhesion of the optical resin layer and the bonding layer along theinterface of these two layers, they come to be delaminated from eachother. Therefore, it is important to increase the adhesion force alongthe interface of the two layers and reduce the tensile stress there toprevent delamination of the optical resin layer and the bonding layerfrom each other.

For instance, in an optical element disclosed in Japanese PatentApplication Laid-Open No. 2016-194609, the base member is made to havean outer peripheral zone that shows a small thickness. With such anarrangement, when tensile stress arises along the interface of the resinlayer and the bonding layer, the base members will easily be deformed(bent) by the stress to in turn prevent the resin layer and the bondinglayer from being delaminated from each other. However, as the thicknessof the base members is reduced at the outer peripheral zone thereof, theperipheral areas of the base members that cannot be utilized asoptically effective regions increase to give rise to a problem that theouter diameter of the optical element inevitably needs to be increased.

In contrast, a composite optical element according to the presentinvention that has the above-described configuration can minimize theincrease of the outer peripheral areas that cannot be utilized asoptically effective regions so that the present invention provides acomposite optical element that shows excellent environmental resistanceeven when it has a small outer diameter.

The present invention provides a composite optical element comprising: afirst base member having a light entering/exiting surface extending froma center, the center being aligned with the optical axis of the opticalelement, to an outer periphery thereof; an optical resin layer extendingfrom the center to an outer periphery thereof; a bonding layer extendingfrom the center to an outer periphery thereof; and a second base memberhaving a light entering/exiting surface extending from the center to anouter periphery thereof; the first base member, the optical resin layer,the bonding layer and the second base member being sequentiallylaminated in the above-described order; the optical resin layer and thebonding layer being sandwiched between the light entering/exitingsurface of the first base member and the light entering/exiting surfaceof the second base member, wherein the bonding layer has a thicknesswhich changes along a straight line extending from the center toward theouter periphery of the bonding layer, the thickness being greater at anintermediate position located between a first position and a secondposition than at the first position and at the second position, thefirst position corresponding to a point apart from the center by 0.8times a distance to the outer periphery of the optical resin layer fromthe center, the second position corresponding to the outer periphery ofthe bonding layer.

Firstly, in a composite optical element according to the presentinvention, the outer diameter of the optical resin layer is made greaterthan the outer diameter of the bonding layer and there exists no layerthat is formed to adhere to the two base members. Then, as a result,when expansions and compressions take place due to changes in theenvironmental temperature, there does not arise any stress thatconcentratedly appears in one single layer of the composite opticalelement. Therefore, the risk that the layers are delaminated from eachother is minimized.

Additionally, in a composite optical element according to the presentinvention, the thickness He of the bonding layer on the circle centeredat the optical axis of the optical element with a diameter equal to 0.8ϕr as measured on a straight line extending from the center to the outerperipheral edge thereof, which diameter also corresponds to the outerdiameter of the region that is optically utilized (optically effectiveregion), and the thickness Ha of the bonding layer at spots that may ormay not continuously arranged on the circle also centered at the opticalaxis of the optical element with a diameter equal to ϕa, which circle isfound outside the above-defined circle and in the region that isoptically not utilized, satisfies the relationship requirement of He<Ha.Since the bonding layer is not optically utilized but arranged only tomake the optical resin layer and the second base member adhere to eachother, He is required to be made as small as possible (and the bondinglayer is required to be made as thin as possible in the opticallyeffective region). If He has a large value (and hence the bonding layerhas a large thickness in the optically effective region) and theenvironmental temperature changes, the composite optical element isdeformed and the refractive index of the composite optical elementfluctuates to a large extent due to the thermal expansion of the bondinglayer to consequently degrade the performance of the optical element. Onthe other hand, since Ha is made greater than He (and hence thethickness of the bonding layer in the region outside the opticallyeffective region is made greater than the thickness of the bonding layeron the circle having a diameter equal to the outer diameter of theoptically effective region), the optical performance of the opticalelement is maintained to its original high level if the environmentaltemperature changes and any risk that the layers are delaminated fromeach other is minimized. This is because the modulus of elasticity ofthe bonding layer is very small relative to the modulus of elasticity ofthe base members and that of the optical resin layer and hence thestress that arises in the bonding layer is reduced and/or widelydispersed. Additionally, since the circle whose diameter is equal to ϕais found in the region that is not optically utilized, the bonding layerdoes not adversely affect the performance of the optical element even ifthe bonding layer is made to have a large thickness on the circle.

Furthermore, in a composite optical element according to the presentinvention, the thickness Hb of the bonding layer on the circle having adiameter equal to ϕb, which diameter ϕb is not greater than the outerdiameter ϕg of the bonding layer, and which circle is located outsidethe circle having the diameter equal to ϕa, satisfies the relationshiprequirement of Ha>Hb, where Ha is the thickness of the bonding layer onthe circle having a diameter equal to ϕa. Note that the thickness of thebonding layer continuously changes from the circle having the diameterof ϕa to the outer peripheral edge (having the outer diameter of ϕg).Therefore, the thickness Ha of the bonding layer on the circle havingthe diameter of ϕa is greater than the thickness of the bonding layer atthe outer peripheral edge thereof. The thickness of the bonding layer isrequired to be made as large as possible on the circle having a diameterthat is equal to ϕa in order to disperse the stress and minimize therisk of delamination. However, if the thick part of the bonding layercontinues from the circle having the diameter of ϕa to the outerperipheral edge of the bonding layer that has the diameter of ϕg,moisture can penetrate into the optical element from the outside by alarge amount to give rise to a dilation deformation and a reduction ofthe adhesion on the interface. Then, delamination of the layers of thecomposite optical element can occur frequently particularly when theoptical element is operated for a long period of time because themodulus of elasticity of the bonding layer has a small value andmoisture can easily pass through the bonding layer. Thus, thepenetration of moisture is reduced and the delamination risk isminimized by reducing the thickness of the bonding layer on a circlethat is located outside the circle having the diameter of ϕa and whosediameter is equal to 0, which diameter ϕb is not greater than the outerdiameter ϕg of the bonding layer. Incidentally, while in the above, thepositions corresponding to the diameters ϕa, ϕb and ϕg are assumed to belocated at the same distance from the center in all the directions (i.e.located on a circle) for the convenience of explanation, they may be notexactly the same for all the directions. However, it is preferable toprovide the portion having an increased thickness at substantially thesame distance for all the directions (i.e. in a ring) because suchconstitution will disperse stress uniformly.

Note that the effect of preventing delamination from taking place canmore reliably be obtained when the thickness of the bonding layersatisfies the requirement of 1.2He<Ha and Ha is greater than 1.2 timesof the thickness of the bonding layer at the outer peripheral edgethereof. This is because the stress dispersing effect of the bondinglayer is further improved when Ha is greater than 1.2 times of He andthe amount of moisture penetrating into the bonding layer from theoutside can be reduced more effectively when Ha is greater than 1.2times of the thickness of the bonding layer at the outer peripheral edgethereof (and hence the thickness of the bonding layer is further reducedfrom Ha toward the outer peripheral edge of the bonding layer).

Furthermore, the effect of preventing delamination from taking place canmore reliably be obtained when the thickness of the bonding layer is notsmaller than 1 μm and not greater than 100 μm at any arbitrarilyselected point thereof. In other words, when the smallest value of thethickness of the bonding layer is smaller than 1 the bonding layer maysometimes not satisfactorily exert its stress dispersing effect. When,on the other hand, the largest value of the thickness of the bondinglayer is greater than 100 the bonding layer expands and contracts to alarge extent in response to changes in the environmental temperature togive rise to a risk of adversely affecting the performance of theoptical element.

Additionally, the effect of preventing delamination from taking placecan still more reliably be obtained when the modulus of elasticity ofthe bonding layer is not greater than ⅕ of the modulus of elasticity ofthe optical resin layer. On the other hand, the stress dispersing effectis not satisfactory and the effect of preventing delamination fromtaking place can hardly be obtained when the modulus of elasticity ofthe bonding layer is greater than ⅕ of the modulus of elasticity of theoptical resin layer.

Now, the present invention will be described by referring to theaccompanying drawings that illustrate currently preferable embodimentsof the invention. Note, however, that the present invention is by nomeans limited by the embodiments that will be described below.

[Optical Element]

FIG. 1 is a schematic cross-sectional view of an embodiment of opticalelement according to the present invention. The optical element of thisembodiment comprises a first base member 1 having a lightentering/exiting surface showing a convex profile, an optical resinlayer 2 formed on the light entering/exiting surface showing a convexprofile of the first base member 1 and having an uneven film thicknessof decreasing from the center toward the outer periphery thereof, abonding layer 3 showing a thin film-like appearance and having asubstantially uniform film thickness and a second base member 4 bondedto the bonding layer and having a light entering/exiting surface showinga concave profile, the layers being laminated in the above-mentionedorder. Light may be made to enter the optical element from eithersurface side thereof. In other words, light may be made to enter theoptical element from the side of the first base member 1 or from theside of the second base member 4.

Materials that can be used to form the first base member 1 and thesecond base member 4 include not only popular optical glass materialssuch as silicate glass, borosilicate glass and phosphate glass but alsovarious glass materials such as quartz glass and glass ceramic.Additionally, not only glass materials but also light transmitting(hard) plastic materials can also be used to form the first base member1 and the second base member 4. The radius of curvature of the convexprofile and that of the concave profile can appropriately be selectedaccording to the desired optical performance of the optical element.

The optical resin layer 2 is arranged on the first base member 1. As forthe profile of the optical resin layer 2, the optical resin layer 2 hasan uneven film thickness that is largest at and near the center thereofand continuously decreasing toward the outer peripheral edge thereof.The film thickness of the optical resin layer 2 is the thickness of theoptical resin layer 2 in the surface normal direction relative to thesurface of the first base member 1 on which the optical resin layer 2 isformed.

An appropriate material can be selected for the optical resin layer 2according to the desired optical characteristics of the optical elementand those of the optical resin layer 2. Examples of materials that canbe used for the optical resin layer 2 include thermosetting resinmaterials and light curable resin materials. Thermosetting resinmaterials and light curable resin materials that can be used to form theoptical resin layer 2 may contain micro particles in a dispersed statefor the purpose of adjusting the optical characteristics of the opticalresin layer 2. From the viewpoint of adoptability of a simplemanufacturing process, the use of a light curable resin material ispreferable. From the viewpoint of capability of obtaining excellentoptical characteristics, the use of an acrylic resin material ispreferable. Acrylic resin materials that can be used to form the opticalresin layer 2 include those obtained by polymerizing or copolymerizing acompound having one or more acryloyl groups and/or methacryloyl groupsand expressed by the chemical formula (1) shown below.

In the chemical formula (1), each of X and Y is a substituent selectedfrom the substituents listed below.

(where * represents atomic bonding with R₁ or R₂)

In the chemical formula (1), each of R₁ and R₂ represents a substituentselected from a hydrogen atom, an alkyl group having one or two carbonatoms and a (meth)acryloyl group (referring to an acryloyl group or amethacryloyl group hereinafter) and each of Z₁ and Z₂ represents asubstituent selected from a hydrogen atom, a halogen atom, an alkoxygroup having one or two carbon atoms, an alkylthio group having one ortwo carbon atoms, an unsubstituted alkyl group having one or two carbonatoms and substituents expressed by the chemical formula (2) shownbelow.

(where ** represents atomic bonding and m represents 0 or 1, while nrepresents an integer from 2 to 4 and R represents a hydrogen atom or amethyl group.)

In the chemical formula (1), each of a and b represents an integer from0 to 2. When a is 2, the two Z₁s may be the same or different from eachother. When b is 2, the two Z₂s may be the same or different from eachother.

The smallest film thickness of the optical resin layer 2 is not lessthan 10 μm and the largest film thickness thereof is not more than 3 mm.In other words, the film thickness of the optical resin layer 2 isdesirably not less than 10 μm and not more than 3 mm. When the filmthickness is less than 10 μm, the chromatic aberration correctingfeature of the optical element 1 may not be satisfactory. When, on theother hand, the film thickness exceeds 3 mm, stress distribution canarise in the film thickness direction to give rise to cracks. The filmthickness of the optical resin layer 2 may typically be 1 mm at thecenter and 100 μm at the outer peripheral edge thereof.

The bonding layer 3 is a layer that is arranged on the optical resinlayer 2 so as to bond the optical resin layer 2 and the second basemember 4 together. The bonding layer 3 is made of a resin material thatis selected to maximize the force that bonds the optical resin layer 2and the second base member 4 together. The type of the resin material ofthe bonding layer 3 is not subject to any particular limitations and maybe a thermosetting resin material or a light curable resin material.However, the use of a light curable resin material is preferable becausea simple manufacturing method can be used for manufacturing the opticalelement and the bonding layer is not remarkably deformed during theoptical element manufacturing process when alight curable resin materialis selected for the bonding layer 3.

The modulus of elasticity of the bonding layer 3 is desirably remarkablysmaller than the modulus of elasticity of the optical resin layer 2.More specifically, the modulus of elasticity of the bonding layer 3 isdesirably less than ⅕ of the modulus of elasticity of the optical resinlayer 2. When the modulus of elasticity of the bonding layer 3 isremarkably smaller than the modulus of elasticity of the optical resinlayer 2, the stress that can arise in the optical resin layer 2 and thebonding layer 3 can be reduced if they are deformed due to changes inthe environmental temperature. The modulus of elasticity of the bondinglayer 3 is preferably not less than 100 MPa and not more than 1 GPa.

The film thickness of the bonding layer 3 is preferably within the rangebetween 1 μm and 100 μm. The film thickness of the bonding layer 3 isthe thickness of the bonding layer 3 in the surface normal directionrelative to the surface of the bonding layer 3 where it contacts thesecond base member 4. When the (film) thickness of the bonding layer 3is less than 1 the stress dispersing effect of the bonding layer 3cannot satisfactorily be obtained. When, on the other hand, the (film)thickness of the bonding layer 3 exceeds 100 μm, the bonding layer 3expands and contracts remarkably in response to changes in theenvironmental temperature to give rise to a risk of adversely affectingthe performance of the optical element.

FIG. 2 is a schematic cross-sectional view of a peripheral part of acomposite optical element according to the present invention. While theouter diameter of the first base member 1 and that of the second basemember 4 are not subject to any particular limitations, they aretypically not less than 5 mm and not more than 100 mm. The outerdiameter ϕr of the optical resin layer 2 is smaller than the outerdiameter of the first base member 1 and preferably not less than 90% andnot more than 99% of the outer diameter of the first base member 1. Theouter diameter ϕg of the bonding layer 3 is smaller than the outerdiameter ϕr of the optical resin layer 2 and preferably not less than90% and not more than 99% of ϕr. The diameter ϕa and the diameter ϕB(ϕa<ϕb) are so selected that they are greater than 0.8 ϕr, which is theouter diameter of the optically utilizable region of the opticalelement, and do not exceed the outer diameter ϕg of the bonding layer 3and that, when the thicknesses of the bonding layer 3 on the respectivecircles with diameters of 0.8 ϕr, ϕa and ϕb are He, Ha and Hb, thebonding layer 3 is so formed that it satisfies the requirement of He<Haand also the requirement that Ha is greater than the thickness of thebonding layer 3 at the outer peripheral edge thereof. Note that thevalue of ϕa is found between the value of 0.8 ϕr and the value of ϕb andpreferably not less than 81% and not more than 95% of the value of ϕr.Additionally, the value of ϕb is found between the value of ϕa and thevalue of ϕg (and may be equal to ϕg) and preferably not less than 85%and not more than 98% of the value of ϕr. Each of He, Ha and Hb ispreferably not less than 1 μm and not more than 100 Furthermore,preferably the requirement of 1.2<Ha and also the requirement that Ha isgreater than 1.2 times of the thickness of the bonding layer 3 at theouter peripheral edge thereof are satisfied.

While the film thicknesses and the profile of the bonding layer 3 thatsatisfy the above-described requirements can be obtained by processingthe surface of the second base member 4 that contacts the bonding layer3, they can more easily be obtained by processing the metal mold at thepart thereof that defines the surface of the optical resin layer 2 to bebrought into contact with the bonding layer 3 and subsequentlytransferring the profile of the processed metal mold onto the opticalresin layer 2.

[Manufacturing of Optical Element]

While there are no particular limitations to the method of manufacturingan optical element according to the present invention, an exemplarmethod of manufacturing a composite optical element (lens) comprising apair of glass-made base members, an optical resin layer 2 and a bondinglayer 3, of which the optical resin layer 2 and the bonding layer 3 areformed by using respective light curable materials and sandwichedbetween the pair of base members 1 and 4, will be described below.

While the profile of each of the glass-made base members is not subjectto any particular limitations, when the composite optical element to bemanufactured is a lens, a plate-shaped member having a lightentering/exiting surface selected from a concave surface, a convexsurface, an axisymmetric aspherical surface, a flat surface or the likeat one or both of the surfaces thereof can be employed for each of them.While the profile of each of the glass-made base members may be selectedfrom various possible profiles, the use of a circular profile ispreferable when the composite optical element to be manufactured is alens. The glass-made base members can be manufactured by cutting,grinding, polishing, press molding or the like.

The glass-made base members are preferably preprocessed on the surfacesthereof to be brought into tight contact respectively with the opticalresin materials. For the purpose of preprocessing the glass surfaces, acoupling process using any of various silane coupling agents having goodaffinity to resin materials can suitably be employed. Specific examplesof coupling agents that can be used for the preprocessing includehexamethyl disilazane, methyl trimethoxy silane, trimethyl chlorosilaneand triethyl chlorosilane.

To begin with, a light curable resin material 6 is placed between thefirst glass-made base member 1 and a metal mold 5 for molding as shownin FIG. 3A. The metal mold 5 to be used here has a surface showing theinverted profile of the intended surface profile of the optical element.For example, such an inverted profile can be produced by cutting themetal mold 5 by means of a precision processing machine. The metal moldis typically prepared by plating a metal base material, which basematerial may be stainless steel or steel, by NiP plating or oxygen-freecupper plating. Then, the light curable resin material 6 and the firstglass-made base member 1 on the metal mold 5 is irradiated with UV raysfrom the side of the first glass-made base member 1 by means of a UV raysource 7 to set the light curable resin material 6 as shown in FIG. 3B.Thereafter, an intermediate product of optical element, in which anoptical resin layer 2 is laid on and integrated with the firstglass-made base member 1, can be obtained by releasing the cured lightcurable resin material 6 from the metal mold 5 as shown in FIG. 3C.Subsequently, as shown in FIG. 3D, an adhesive agent 8 for forming theoptical element is placed between the optical resin layer of theobtained intermediate product of optical element and the secondglass-made base member 4 for molding. Then, all the components of thecomposite optical element that are put together are irradiated with UVrays from the side of the second glass-made base member 4 by means ofthe UV ray source 7 to set the adhesive agent 8, as shown in FIG. 3E. Asa result, the composite optical element in which the first glass-madebase member 1, the optical resin layer 2, the bonding layer 3 and thesecond glass-made base member 4 are subsequently laid in theabove-mentioned order as shown in FIG. 1 is obtained. Aspost-processing, a thermal annealing operation, an operation ofadditional irradiation of UV rays, a heating and/or UV rays irradiatingoperation in an oxygen-free environment and/or some other operation maybe executed for the purpose of relaxation of the residual stress afterthe molding process.

[Optical Devise]

Specific examples of application of a composite optical elementaccording to the present invention include a lens for forming an opticaldevise (imaging optical system) to be used for a camera or a videocamera and a lens for forming an optical devise (projection opticalsystem) to be used for a liquid crystal projector. Such optical systemscomprise a casing and a plurality of lenses arranged in the casing and acomposite optical element according to the present invention may be usedfor at least one of those plurality of lenses.

[Imaging Apparatus]

FIG. 4 is a schematic cross-sectional view of an embodiment of imagingapparatus suitably formed by using a composite optical element accordingto the present invention, showing the configuration thereof. Thisembodiment is a digital single-lens reflex camera 600. While a cameramain body 602 and a lens barrel 601, which is an optical apparatus, areconnected to each other in FIG. 4, the lens barrel 601 is a so-calledinterchangeable lens barrel that is detachably connected to the cameramain body 602.

The rays of light coming from the target of shooting by the camera 600are picked up by the optical system including a plurality of lenses 603,605 arranged on the optical axis of the imaging optical system in acasing 620 of the lens barrel 601. An optical element according to thepresent invention can be used, for example, for the lens 603 and alsofor the lens 605. Note that the lens 605 is supported by an innercylinder 604 such that the inner cylinder 604 supporting the lens 605can be moved relative to the outer cylinder of the lens barrel 601 forthe purpose of focusing and zooming.

In the observation period that comes before a target shooting operation,the rays of light coming from the target are reflected by a main mirror607 in the casing 621 of the camera main body 602 and, after passingthrough the prism 611, produce the image to be picked up by the camerashooting person by way of a finder lens 612. The main mirror 607 maytypically be a half mirror and the rays of light that pass through themain mirror are reflected toward an AF (autofocus) unit 613 by asub-mirror 608 and typically employed for ranging. The main mirror 607is mounted on and bonded to a main mirror holder 640 so as to besupported by the main mirror holder 640. For a target shootingoperation, the main mirror 607 and the sub-mirror 608 are moved awayfrom the light path by means of a drive mechanism (not shown) and ashutter 609 is opened to allow an image pickup element 610 to receivethe rays of light that entered from the lens barrel 601 and passed theimaging optical system and form an optical image to be picked up. Anaperture 606 is arranged to vary the aperture area and adjust thebrightness and the depth of focus in the shooting operation.

While an imaging apparatus formed by using an optical element accordingto the present invention is described above in terms of a digitalsingle-lens reflex camera, an optical element according to the presentinvention can also be used for a smartphone, a compact digital camera orthe like.

EXAMPLES

Now, the present invention will be described further by way of examples.

In each of the following examples, an optical element according to thepresent invention was prepared by means of the method that will bedescribed below and the obtained optical element was evaluated foroptical performance and environment resistance by the method that willalso be described below.

Example 1

To begin with, the method of manufacturing an optical element asillustrated in FIG. 1 will be described below. A piece of glass material(S-FPM2: trade name, available from OHARA) was processed to make it showa biconvex spherical profile with ϕ42 mm and the processed piece wasemployed for the first base member 1. The spherical profile of each ofthe opposite surfaces of the first base member 1 was made to show aradius of 78.0 mm. A piece of glass material (S-NBH53: trade name,available from OHARA) was processed to make it show a biconcavespherical profile with ϕ39 mm and the processed piece was employed forthe second base member 4. The spherical profile of each of the oppositesurfaces of the second base member 4 was made to show a radius of 57.0mm. The metal mold 5 (see FIGS. 3A and 3B) of this example was preparedby oxygen-free copper plating a stainless steel-made base member andsubsequently cutting it by means of a diamond bit. It showed theinverted profile of the intended profile of the optical resin layer 2.The profile of the optical resin layer 2 was such that the filmthickness of the optical resin layer 2 at the center thereof was 1.00 mmand the smallest film thickness of the optical resin layer 2 at theperipheral edge thereof was 0.05 mm, while the distance from the centerto the outer peripheral edge was 20.0 mm.

Firstly, a light curable resin material 6 (see FIGS. 3A and 3B) wasapplied to the first glass-made base member 1 and also to the metal mold5 by means of a dispenser (SMP-3: trade name, available from MUSASHIEngineering). An acrylic resin material whose modulus of elasticityafter curing was 3.5 GPa was employed for the light curable resinmaterial. Subsequently, a jig for accurately setting up the positionalrelationship between the first glass-made base member 1 and the metalmold 5 was prepared and the first glass-made base member 1 and the metalmold 5 were placed in position. Then, the first glass-made base member 1was moved closer to the metal mold 5 to fill the gap between them with alight curable resin material such that the distance from the center tothe outer periphery of the resin material layer becomes 20.0 mm.Thereafter, the resin material was irradiated with UV rays through thefirst glass-made base member 1 from a UV ray source 7 to form theoptical resin layer 2 on the first glass-made base member 1. UV rayswere irradiated with a dose of 10 J. Then, the optical resin layer 2 wasreleased from the metal mold 5.

After releasing the optical resin layer 2 from the metal mold 5, UV rayswere irradiated onto the optical resin layer 2, while the optical resinlayer 2 was being vacuum-heated under the conditions of a degree ofvacuum of 10 Pa and a temperature of 90° C. for two hours, in order toaccelerate the curing of the optical resin layer 2. UV rays wereirradiated with a dose of 10 J. The optical resin layer 2 showed athickness of 1 mm at the center thereof and a thickness of 100 μm at theouter peripheral edge thereof.

Then, a photosetting adhesive agent 8 (see FIGS. 3D and 3E) was appliedto both the optical resin layer 2 and the second glass-made base member4 by means of the dispenser. An adhesive agent (WR8807LK: trade name,available from KYORITSU CHEMICAL) whose modulus of elasticity aftercuring was 0.17 GPa was employed for the adhesive agent 8. Subsequently,the second glass-made base member 4 was arranged oppositely relative tothe optical resin layer 2 and moved closer to the optical resin layer 2and the photosetting adhesive agent 8 was filled into the gap betweenthe second glass-made base member 4 and the optical resin layer 2 tomake the photosetting adhesive agent 8 show a thickness of 0.02 mm atthe center thereof. Thereafter, the adhesive agent 8 was set byirradiating UV rays from the side of the second glass-made base member 4to produce the bonding layer 3. UV rays were irradiated with a dose of10 J. Finally, the first glass-made base member 1, the optical resinlayer 2, the bonding layer 3 and the second glass-made base member 4that were put together were put into an oven and heated at a temperatureof 100° C. for two hours to produce the finished composite opticalelement of Example 1.

FIG. 2 shows the profile of the optical element and the thickness of thebonding layer 3. Referring to FIG. 2, ϕa, ϕb, ϕg and ϕr wererespectively 36.0 mm, 38.0 mm, 38.6 mm and 40.0 mm and He, Ha and Hbwere respectively 0.020 mm, 0.060 mm and 0.030 mm. The film thicknesswas found to be continuously decreasing from Hb toward the outerperipheral edge.

Example 2

The optical element of Example 2 was prepared as in Example 1 exceptthat He was made to be equal to 0.020 mm, Ha was made to be equal to0.024 mm and Hb was made to be equal to 0.020 mm for the film thicknessof the bonding layer 3. Additionally, the film thickness of the bondinglayer 3 was found to be continuously decreasing from Hb toward the outerperipheral edge thereof.

Example 3

The optical element of Example 3 was prepared as in Example 1 exceptthat He was made to be equal to 0.020 mm, Ha was made to be equal to0.100 mm and Hb was made to be equal to 0.030 mm for the film thicknessof the bonding layer 3. Additionally, the film thickness of the bondinglayer 3 was found to be continuously decreasing from Hb toward the outerperipheral edge thereof.

Example 4

The optical element of Example 4 was prepared as in Example 1 exceptthat an acrylic resin material whose modulus of elasticity was made tobe equal to 0.9 GPa was employed for the optical resin layer 2.Additionally, the film thickness of the bonding layer 3 was found to becontinuously decreasing from Hb toward the outer peripheral edgethereof.

Comparative Example 1

The optical element of Comparative Example 1 was prepared as in Example1 except that He was made to be equal to 0.020 mm and the film thicknessoutside He was made to be constant and equal to 0.020 mm so that Ha andHb were non-existent.

Comparative Example 2

The optical element of Comparative Example 2 was prepared as in Example1 except that He was made to be equal to 0.020 mm, Ha was made to beequal to 0.080 mm and the film thickness outside Hb was made to beconstant and equal to 0.080 mm so that Hb was non-existent.

Comparative Example 3

The optical element of Comparative Example 3 was prepared as in Example1 except that He was made to be equal to 0.080 mm and the film thicknessoutside He was constantly less than He so that Ha was non-existent,while Hb was made to be equal to 0.020 mm. Additionally, the filmthickness of the bonding layer 3 was found to be continuously decreasingfrom Hb toward the outer peripheral edge thereof.

Comparative Example 4

The optical element of Comparative Example 4 was prepared as in Example1 except that He, Ha and Hb were respectively made to be equal to 0.020mm, 0.150 mm and 0.030 mm. Additionally, the film thickness of thebonding layer 3 was found to be continuously decreasing from Hb towardthe outer peripheral edge thereof.

Comparative Example 5

The optical element of Comparative Example 5 was prepared as in Example1 except that ϕg was made to be equal to 39.0 mm and ϕr was made to beequal to 38.6 mm for the profile of the optical element. Additionally,the film thickness of the bonding layer 3 was found to be continuouslydecreasing from Hb toward the outer peripheral edge thereof.

The method of evaluating the optical elements prepared in the examplesand the comparative examples will be described below.

[Evaluation of Optical Performance]

Imaging lenses were prepared respectively by incorporating the preparedoptical elements into optical systems and connected to a camera on a oneby one basis and a plate that bore stripes of three colors of RGB wasshot by the camera for each of the colors to obtain a photograph of theplate. Each of the taken pictures was compared with the plate and thevalue of the resolution of the color of the picture was measured bymeans of image processing software. When the measured chromaticaberration exceeded the predetermined reference value, rating of B wasgiven to the optical performance of the optical element employed to takethe picture. When, on the other hand, the measured chromatic aberrationwas less than the predetermined reference value, rating of A was givento the optical performance of the optical element.

[Evaluation of Environmental Resistance]

Each of the prepared optical elements was put into a constanttemperature oven, in which the temperature was held at 80° C., for twohours. Subsequently, the optical element was taken out from the constanttemperature oven and left still for a period of not less than two hours.Thereafter, if there was any mutual separation of the optical resinlayer 2 and the bonding layer 3 at the peripheral edge of the opticalelement or not was observed at room temperature (23° C.±2° C.) by way ofan optical microscope. When mutual separation of the optical resin layer2 and the bonding layer 3 was observed, rating of B was given to theenvironmental resistance of the optical element. When, on the otherhand, no mutual separation of the optical resin layer 2 and the bondinglayer 3 was observed, rating of A was given to the environmentalresistance of the optical element.

The manufacturing conditions and the results of the evaluations of theprepared optical elements were listed in Table 1 shown below.

TABLE 1 Modulus of elasticity (GPA) Optical Evaluation resin BondingOptical element profile (mm) Optical Environmental Total layer layer 0.8ϕr ϕa ϕb ϕg ϕr He Ha Hb performance resistance rating Example 1 3.5 0.1732.0 36.0 38.0 38.6 40.0 0.020 0.060 0.030 A A A Example 2 3.5 0.17 32.036.0 38.0 38.6 40.0 0.020 0.024 0.020 A A A Example 3 3.5 0.17 32.0 36.038.0 38.6 40.0 0.020 0.100 0.030 A A A Example 4 0.9 0.17 32.0 36.0 38.038.6 40.0 0.020 0.060 0.030 A A A Comparative 3.5 0.17 32.0 — — 38.640.0 0.020 — — A B B Example 1 Comparative 3.5 0.17 32.0 36.0 — 38.640.0 0.020 0.080 — A B B Example 2 Comparative 3.5 0.17 32.0 — 38.0 38.640.0 0.080 — 0.020 B A B Example 3 Comparative 3.5 0.17 32.0 36.0 38.038.6 40.0 0.020 0.150 0.030 B A B Example 4 Comparative 3.5 0.17 32.036.0 38.0 39.0 38.6 0.020 0.060 0.030 A B B Example 5

Rating of A was given to an optical element that obtained rating of Afor both of the optical performance and the environmental resistance.Rating of B was given to an optical element that obtained rating of Bfor either of the optical performance and the environmental resistance.As clearly seen from Table 1, the present invention provided opticalelements that had a small outer diameter and showed excellentenvironmental resistance.

Effect of the Invention

Thus, the present invention can provide an optical element that has asmall outer diameter and still shows excellent environmental resistance.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Applications No.2019-015785, filed Jan. 31, 2019, and No. 2019-222942, filed Dec. 10,2019, which are hereby incorporated by reference herein in theirentirety.

What is claimed is:
 1. A composite optical element comprising: a firstbase member having a light entering/exiting surface extending from acenter, the center being aligned with the optical axis of the opticalelement, to an outer periphery thereof; an optical resin layer extendingfrom the center to an outer periphery thereof; a bonding layer extendingfrom the center to an outer periphery thereof; and a second base memberhaving a light entering/exiting surface extending from the center to anouter periphery thereof; the first base member, the optical resin layer,the bonding layer and the second base member being sequentiallylaminated in the above-described order; the optical resin layer and thebonding layer being sandwiched between the light entering/exitingsurface of the first base member and the light entering/exiting surfaceof the second base member, wherein the bonding layer has a thicknesswhich changes along a straight line extending from the center toward theouter periphery of the bonding layer, the thickness being greater at anintermediate position located between a first position and a secondposition than at the first position and at the second position, thefirst position corresponding to a point apart from the center by 0.8times a distance to the outer periphery of the optical resin layer fromthe center, the second position corresponding to the outer periphery ofthe bonding layer.
 2. The composite optical element according to claim1, wherein the thickness of the bonding layer at the intermediateposition is greater than 1.2 times the thickness at the first positionand is greater than 1.2 times the thickness at the second position. 3.The composite optical element according to claim 1, wherein the firstposition and the intermediate position are respectively provided in aring.
 4. The composite optical element according to claim 1, wherein thefirst position and the intermediate position are located outside theoptically effective region of the composite optical element.
 5. Thecomposite optical element according to claim 1, wherein the thickness ofthe bonding layer is within a range not smaller than 1 μm and notgreater than 100 μm.
 6. The composite optical element according to claim1, wherein the modulus of elasticity of the bonding layer is not greaterthan ⅕ of the modulus of elasticity of the optical resin layer.
 7. Thecomposite optical element according to claim 1, wherein the opticalresin layer has an uneven thickness with a smallest value of not lessthan 10 μm and a largest value of not more than 3 mm.
 8. The compositeoptical element according to claim 1, wherein the thickness of thebonding layer continuously changes from the intermediate position to thesecond position.
 9. An optical apparatus comprising a casing and anoptical system including a plurality of lenses arranged in the casing,wherein at least one of the plurality of lenses is a composite opticalelement according to claim
 1. 10. An imaging apparatus comprising acasing, an optical system including a plurality of lenses arranged inthe casing, and an image pickup element for receiving light afterpassing through the optical system, wherein at least one of theplurality of lenses is a composite optical element according to claim 1.11. The imaging apparatus according to claim 10, wherein the imagingapparatus is a camera.